Two blade or Three?

[RV7Ron]

Hey guys, new to the forums, soon to be -7 builder...visiting Vans on April 10 and plopping down a check for the empennage.

I plan to put a IO-390 out front with a C/S prop and fly out of Denver. What are the performance advantages and disadvantages between a two-bladed prop and a three? We can ignore the added cost/maintenance side of it for this discussion as I am mostly concerned about performance pros/cons. Does a three have better climb performance and reduced performance in cruise? I know I have some time to decided on this, just wanted to educate myself with your help.

[f1rocket]

Generally speaking, a three-blade prop is less efficient than a two-blade. If performance is your ONLY concern, then go with a two-blade. However the various three-blade models offer other advantages in terms of vibration, looks, and ground clearance to name just a few. In some instances, the three-blades are nearly the same as two-bladed in terms of speed, but not as fast.

BTW, I have a three-blade prop on my Rocket and I did it for weight, looks, and ground clearance.

[Sid Lambert]

I think it is generally accepted that a 3 blade prop has better climb but slightly less top end than a 2 bladed prop. There of course could be exceptions to the rule.

Saying that, even flying from Leadville, CO on a hot day with any CS RV loaded to max gross should be a none event.

[Norman CYYJ]

Three blades are also quieter than two blades.

[RV7Ron]

Thanks guys, that helps...I didnt consider some of the other things you mentioned...less vibration, quieter, more ground clearance. I do like the looks of them too.

Randy, what kind of three blade do you have on your Rocket?

[osxuser]

Vibration is definitely different (less is relative), Quieter is most a function of tip speed, i've flown with some real quiet 2-blades at 2200RPM (Cessna 182 for instance). Our 3-blade on the Cardinal is actually noiser at RPM than the older -7666 2-blade, but as soon as the rpms are down to 2300 it shines, 2500+ is a real racket though.

I'd say with the -390 the best combo is going to be with a Blended airfoil Hartzell, but if you want to take the cruise hit, a 3-blade MT will work too. It WILL reduce you top speed significantly, and hasn't been proven to climb better.

Anyone know if the WW-200 is approved for the -390 yet? I think it was HP limited to 200?

[f1rocket]

Originally Posted by RV7Ron

Thanks guys, that helps...I didnt consider some of the other things you mentioned...less vibration, quieter, more ground clearance. I do like the looks of them too.

Randy, what kind of three blade do you have on your Rocket?

I have the MT.

[breister]

There may be one other esoteric consideration. Some aircraft cowlings are engineered so that the prop pulse coincides with induction, theoretically providing slightly higher MP in flight. For a 4-banger, this requires a 2 bladed prop to work.

But then, I haven't really ever heard of someone switching to a 3 bladed prop and noticing a measurable loss of MP - so it may be B.S.

[steveKs.]

My RV8 has a three blade Hartzell and I like it for ground clearance and noise. I can't tell that the top speed performance is compromised and like all 200 horse RV's it climbs like crazy. I fly with a number of RV8's and 4's with 180-200 hp and I am just as fast or faster. I am fairly light at 1105 lbs.

I don't think you would be giving up anything significant performance wise with the three blade. I am a bit nose heavy but I like that for when I haul big people in the back.

[Geico266]

2 to go, 3 to show.

There is no doubt a 3 blader looks cool on an RV.